I've never been able to make sense of the graph. The actual numbers depend on the image scale but you want the HFR as low as possible and Max as high as possible. Nights with poor seeing can make fine focus a pain to use. I used it for a long time but it always left a nagging doubt as to whether I actually got the lowest HFR possible. A lot of time was spent going in and out of focus trying to minimise HFR.

I've since gone to a Bahtinov mask. There's no comparison. The mask leaves no doubt about when you've reached proper focus.

I use both the Bahtinov mask and Nebulosity's fine focus. The mask gets you on when you have a bright star, but when doing Ha or OIII imaging, and switching filters, I need Nebulosity's fine focus to get the best focus. In good seeing, The FWHM is less than 2 pixels, usually 1.6-1.8 with my 800 mm fl and in bad seeing can be greater than 3.